Color imaging devices use combinations of different device-dependent coordinates to form color images for display or printout on media such as paper or film. Many hardcopy printing devices use combinations of cyan, magenta, yellow, and black (CMYK) to form color imagery. These device-dependent coordinates of C, M, Y and K may be combined to form a gamut of colorimetric values that the device is capable of producing. Display devices, such as cathode ray tubes (CRTs) or flat panel monitors, may use the device-dependent coordinates of red, green, and blue (RGB). Some high-fidelity color imaging devices may use the device-dependent coordinates cyan, magenta, yellow, and black in combination with other coordinates such as orange and green. These and other device-dependent coordinate systems have been developed for use with various color imaging devices.
Many different device-independent coordinate systems have been developed in an attempt to standardize color specification across different devices. For instance, the Commission Internationale de l'Eclairage (CIE) has developed device-independent color spaces such as the L*a*b* color space (hereafter L*a*b* color space, L*a*b* space, or simply L*a*b*) and the XYZ color space (hereafter XYZ color space, XYZ space, or simply XYZ). Moreover, several other organizations and individuals have developed other device-independent color spaces.
A point in a device-independent color space theoretically defines a color value irrespective of any particular device coordinates. A point in L*a*b* space or XYZ space, for instance, can be mapped to a point in a device gamut. That point in the device gamut, in turn, defines the device-dependent coordinates that will theoretically cause the device to produce a color that is visually equivalent to that defined by the point in L*a*b* space or XYZ space.
The term “soft proofing” refers to a proofing process that makes use of a display device rather than a printed hard copy. Traditionally, color proofing techniques have relied on “hard copy proofing,” where proofs are printed out on paper or other print media and inspected to ensure that the images and colors look visually correct. For instance, color characteristics can be adjusted and successive hard copy prints can be examined in the hard copy proofing process. After determining that a particular proof is acceptable, the color characteristics used to make the acceptable proof can be reused to mass-produce, e.g., on a printing press, large quantities of print media that look visually equivalent to the acceptable proof.
Soft proofing is highly desirable for many reasons. For instance, soft proofing can remove the need to print copies of the media during the proofing process. Moreover, soft proofing may allow multiple proofing specialists to proof color images from remote locations simply by looking at display devices, rather than awaiting delivery of hard copies. Soft proofing can be faster and more convenient than hard proofing. Moreover, soft proofing can reduce the cost of the proofing process. For these and other reasons, soft proofing is highly desirable.
A major problem with soft proofing, however, is the difficulty in achieving a good visual match between the colors displayed on the soft proofing display device and the colors that appear on the actual printed hard copy. As mentioned above, device-independent coordinates theoretically standardize color specification. Thus, theoretically the CMYK device coordinates of a hard copy printout could be converted to device-independent coordinates and then converted to RGB device coordinates. Also, theoretically the colors displayed using the RGB device coordinates would be visually equivalent to those of the hard copy print out. In reality, however, the colors that appear on the display may look different than those of the hard copy printout, even though the images displayed by the soft and hard copy media produce substantially identical device-independent values. Soft proofing cannot work effectively, and gain widespread adoption in the industry, if the colors on the soft proofing display device do not provide an acceptable visual match with the colors on hard copy printouts.